The present invention relates to internal bicycle transmissions and more particularly to a mounting system for mounting an internal bicycle transmission to a bottom bracket shell.
Internal bicycle transmissions may be located at the rear wheel hub and/or at the front crank assembly. Internal bicycle transmissions provide multiple transmission paths corresponding to multiple bicycle gear ratios. Typically, an internal bicycle transmission includes a planetary gear mechanism with a sun gear, a ring gear, a plurality of planet gears meshing with the ring gear, and sun gear and a carrier for supporting the planet gears. To shift between the multiple transmission paths, select elements of the planetary gear mechanism may be locked or unlocked. For example, when the sun gear is locked and the planet gears are driven, the ring gear is overdriven. This is referred to as overdrive, where the output at the ring gear rotates faster than the input at the crank assembly. Conversely, when the output is rotated slower than the input, it is referred to as underdrive. During overdrive, the planet gears drive the ring gear forward, but exert an equal but opposite reactive rotational force on the fixed sun gear. This reactive torque may be substantial. When the sun gear is unlocked, it is prevented from carrying a reactive load from the planet gears. In this condition overdrive is prevented, resulting in the planet gears driving the chainring at a 1:1 gear ratio. An advantage of an overdrive system is that a smaller output chainring can be used, saving weight, and providing more clearance between the chainring and the ground. However, a disadvantage of a smaller chainring is the increased likelihood of chain derailment because there are fewer teeth engaging the chain. A chain management system may be used to inhibit chain derailment.
When the internal bicycle transmission is located at the crank assembly, the planetary gear mechanism is operatively connected to the crank assembly. The crank assembly generally includes crank arms attached to a crank shaft extending through a bottom bracket shell of the bicycle frame. The planetary gear mechanism is operatively connected to an input crank arm and an output chainring that transfers the pedaling force to the rear wheel through a bicycle chain. Existing planetary gear mechanism may be mounted to a mounting plate that is axially pinched between the outboard face of the bottom bracket shell and a threaded cap while being rotationally fixed by the frame tubes, or mounted to a mounting plate that is locked to an interior of a modified bottom bracket shell. One disadvantage of these configurations is that additional elements are mounted to the bottom bracket shell, requiring a redesign of the bottom bracket. Another disadvantage of the configuration that uses of the face of the bottom bracket for axially pinching the ground plate, is that it takes up extra space axially, resulting in a longer axle and a wider stance between pedals. Therefore, there is a need for a mounting system that mounts a planetary gear mechanism to the bottom bracket shell without requiring redesign of the bottom bracket.